In a conventional steam turbine electric power plant, steam flowing from a steam generator to a high pressure turbine element is regulated by a main control valve and serially connected throttle valve in order to govern the flow of steam into the high pressure turbine. The normally open control valve is switched from an open to a closed position whenever the turbine reaches a predetermined overspeed condition caused by a sudden loss of load in the generator electrical system or the failure of a component in the steam system. Seating of the main control valve blocks the flow of high temperature steam from the steam generator to the turbine. When the main control valve is suddenly closed, an alternate steam flow path must be provided in order to prevent steam pressure from exceeding maximum limits. A fast opening bypass valve which is quickly responsive to a trip signal is used to redirect the steam flow directly to the condenser whenever the main control valve is closed.
FIG. 1 illustrates in a simplified manner the relationship between a bypass valve (20) and a main control valve (22). The throttle valve can be assumed to be incorporated in the control valve 20. With the bypass valve closed, steam travels from the steam supply system through the control valve (22) and into the high pressure turbine element 24. Steam exits the turbine and passes through condenser 26 before returning to the steam supply system 28 for reheating.
During startup as well as during normal operation it is necessary that the bypass valve be smooth opening and responsive to changes in flow through the turbine. In general, the bypass valve operates in conjunction with the control valve to regulate main steam pressure, i.e., if steam flow through the turbine is reduced, the bypass valve is opened sufficiently to maintain the steam pressure relatively constant at the control valve. The bypass valve is used at start-up to provide the minimum required flow through the steam generator.
While bypass valve design and operation are critical in any steam turbine, such features are particularly important in nuclear steam turbine systems which operate at lower pressure levels than conventional fossil fuel systems and therefore require that relatively higher volumes of steam be supplied to the turbine in order to achieve desired output levels. Therefore, both the control valves and the bypass valves found in nuclear systems are larger than their fossil system counterparts. This sizing requirement of a bypass valve in combination with requirements that it be both fast opening and smoothly controllable has presented some unique problems. For example, many bypass valves have experienced steam leakage while in a closed state. Although such leakage initially has only a minor effect on system performance, continued leakage erodes the valve seat leading to significant system deterioration and requiring valve replacement.
The leakage problem has been determined to be resolvable by increasing the valve seating force at closing. However, the requirements that the valve be fail-safe and be rapid opening prohibit conventional solutions for increasing valve seating force. For example, an increase in spring closing force or use of a double acting hydraulic valve are not appropriate since these solutions have a negative effect on the required fast opening and fail-safe characteristics. Although the leakage problem is believed to be generally common to bypass valves generally, it is believed that the problem also occurs in valves containing flexible seats since such seats exert two variable forces against the closing spring. In such valves, a valve plug presses against a flexible seat causing deflection from a relaxed position to a deflected position. The deflected seat exerts an opposing spring force which increases with the deflection. Furthermore, as the seat deflects and moves away from a relaxed position, a greater surface area of the valve seat becomes exposed to the pressure differential between the valve inlet and valve outlet. As a result, the unbalanced force of the steam against the seat increases as the plug deflects the seat. This force is transferred to the closing spring making it more difficult to properly seat the valve plug. These same effects are present to a lesser degree in fixed valve seats.